The visual inspection or automatic optical inspection has been used for the inspection of coated state and uniform distribution in paints, inks, primers and adhesives and the inspection of applied pattern and finished circuit in the field of a photoresist. The penetrant examination has been applied to a non-destructive penetrant examination of fine defects in the surface of cast articles, machine parts and in the face of weld. A large number of dyes and fluorescent brighteners have been employed as fluorescent detecting agents for those inspections and examinations.
However, dyes and fluorescent brighteners mostly colored with pale yellow to yellow often give undesirable contamination to the material to be inspected. Further, the fluorescent brighteners cannot exhibit sufficient detection effect, since a wavelength of the fluorescent emission is too short. In addition to the above disadvantages of prior fluorescent detecting agents, problem remains that there are no materials satisfying the solubility in organic solvents, the compatibility with resin compositions and the stability of the dispersion when used in the form of fine particles, which are important properties indispensable to practical use. This provides an obstacle to a wide range of applications of fluorescent detecting agents.
For the penetrant examination, for instance, fluorescent detecting agents are used as a solution or dispersion in a penetrant. In the application of those agents to paints, inks, primers and adhesives, they are added in the form dissolved in a solvent or dispersed in fine particles.
In the preparation of a prepreg in the manufacture of a glass epoxy laminate in the field of photoresist, for instance, fluorescent detecting agents are used in the form of fine dispersion or solution together with an ultraviolet light screening material to coat or impregnate in a silane treated glass cloth or nonwoven fabric or they are used in the form dissolved in a resin composition constituting a laminated (epoxy varnish, etc.). When they are used as a fine dispersion, the stability, especially re-cohesiveness of finely divided particles is an important factor in practical use. When they are used as a solution, the solubility and stability in an organic solvent and epoxy varnish are an important factor. At the present time, however, there are no fluorescent detecting agents satisfying those important factors. Thus the application of fluorescent detecting agents has been limited.